cosmic distance ladder

The chain of overlapping methods by which astronomers establish a distance
scale for objects in the universe, from nearby planets to the most remote
quasars and galaxies. At every step of the distance ladder, errors and uncertainties
creep in. Each step inherits all the problems of the ones below, and also
the errors intrinsic to each step tend to get larger for the more distant
objects; thus the spectacular precision at the base of the ladder degenerates
into much greater uncertainty at the very top.

Distances within the Solar System are known to extreme accuracy by a variety
methods, including the motions of the planets in the sky, radar, and timing
of signals from interplanetary probes. Distances to stars within a couple
of thousand light-years come from various geometrical methods; the most
accurate values are those based on measurements of the annual parallax
of about 10,000 nearby stars made by the Hipparcos
satellite. The moving cluster method
can be applied over a similar range, while main-sequence
fitting works with open clusters
out to a distance of about 60,000 light-years.

Beyond the Milky Way Galaxy, distances can be established most reliably
using the period-luminosity
relation of Cepheid variables, backed
by similar observations of other bright stars whose intrinsic brightness
is reasonably well-known, including RR
Lyrae stars and novae. This method can
be applied out to the limit at which Cepheids and other individual stars
can be distinguished inside their host galaxies - up to about 100 million
light-years. For more distant galaxies, standard candles brighter than Cepheids
are needed. These include globular clusters
and Type Ia supernovae, which can be calibrated
as distance indicators using Cepheids in relatively nearby galaxies and
then applied further afield-up to about 200 million light-years for globulars
and out to at least 3 billion light-years for supernovae. At the furthest
limits, only whole galaxies are detectable, so methods such as the Tully-Fisher
relation and Faber-Jackson
relation are used, which link measurable properties of galaxies, or
clusters of galaxies, to their luminosity. Extragalactic distance indicators
enable estimates to be made of the Hubble
constant, a measure of the rate at which the universe as a whole is
expanding. Observation of the redshift
of a remote galaxy or quasar then supplies the object's distance. Over time
the accuracy to which the Hubble constant is known has improved dramatically.
The most recent determination, using data from the Spitzer Space Telescope
has narrowed the uncertainty down to just 3 percent.